Crystallization behavior of linear 1-arm and 2-arm poly(l-lactide)s: Effects of coinitiators

Linear 1-arm and 2-arm poly(l-lactide) [i.e., poly(l-lactic acid) (PLLA)] polymers having relatively low number-average molecular weights (Mn) (≤5×104gmol−1) were synthesized by ring-opening polymerization of l-lactide initiated with tin(II) 2-ethylhexanoate (i.e., stannous octoate) and coinitiators of l-lactic acid, 1-dodecanol (i.e., lauryl alcohol), and ethylene glycol (these PLLA polymers are abbreviated as LA, DN, and EG, respectively). For Mn below 1.5×104gmol−1, non-isothermal crystallization during heating and isothermal spherulite growth were disturbed in linear 2-arm PLLA (EG) compared to those in linear 1-arm PLLA (LA and DN). This finding indicates that the chain directional change, the incorporation of the coinitiator moiety as an impurity in the middle of the molecule, and their mixed effect disturbed the crystallization of linear 2-arm PLLA compared to that of linear 1-arm PLLA, in which the chain direction is unvaried and the coinitiator moiety is incorporated in the chain terminal. Also, the finding strongly suggests that the reported low crystallizability of multi-arm PLLA (arm number≥3) compared to that of linear 1-arm PLLA is caused not only by the presence of branching points but also by the chain directional change, the incorporation of the coinitiator moiety in the middle of the molecule, and their mixed effect. The effects of the chain directional change and the position of the incorporated coinitiator moiety on the crystallization and physical properties of linear 1-arm and 2-arm PLLA decreased with an increase in Mn.